It is only in relatively recent years that society has appreciated the impact and consequences of the fact that we live in a closed ecological system. With an increase in human population and, perhaps more importantly, an increase in industrial activity the effects of ecological changes have become more apparent. One area which has received a great deal of attention is that of water quality, which may be the result of the belated recognition that not only is water of a suitable quality for human consumption a limited resource, but that good water quality is an important, if not critical, factor in the ecological chain. Consequently attention has turned not only to purification of water in local water supplies, but also to limiting the discharge of materials into streams and aquifers generally.
The classes of noxious materials (pollutants) in aqueous discharges vary over an enormously broad spectrum. Among the inorganic pollutants those toxic to a broad spectrum of biological species are especially dangerous. Although heavy metals such as lead, cadmium, and arsenic often are the first culprits thought of, inorganic water soluble cyanide is in a comparably dangerous class because of the generally low tolerance of life forms to cyanide.
The sources of cyanide are many and varied and include iron and steel manufacturing, petroleum and coal pyrolysis processes, the photographic, chemicals, and pharmaceutical industries, precious metal mining and metal finishing, including electroplating and galvanizing. For example, cyanide arises in iron and steel manufacture by reduction of carbonate in the presence of carbon and nitrogen. In power plants coal burning may afford coke oven gas with a hydrogen cyanide concentration on the order of 2 grams per liter. Cyanide solutions are an important component of electroplating and galvanizing, and wash water streams resulting from post-coating treatment often contain significant quantities of cyanide. The widespread prevalence of cyanide in industrial effluents coupled with their near universal toxicity to life has made it imperative to minimize cyanide concentration in aqueous streams.
It appears that the most prevalent methods of cyanide removal are based on the oxidation of cyanide. See generally R. Gierzatowicz et al. Effluent and Water Treatment Journal, 25, 26-31 (1986). Oxidation with chlorine or hypochlorite seems to be industrially the most commonly employed method. The first stage in this oxidation is the formation of cyanogen chloride, ClCN, itself a rather toxic gas, but which is hydrolyzed at a high pH to the less toxic cyanate, CNO. Cyanate is itself hydrolyzed to carbon dioxide and ammonia at low pH, or is further oxidized to carbon dioxide and nitrogen. Another oxidative method uses peroxides such as hydrogen peroxide, Caro's acid, peracetic acid, and so on, as the oxidizing agent. The advantages of this approach vis a vis the chlorine or hypochlorite based process is the lack of toxic byproducts and the formation of environmentally neutral species from the peroxides. A disadvantage is the long reaction times necessary for adequate oxidation. However, cupric ions supposedly act as catalysts for peroxide oxidation. Other oxidizing agents based on Mn(VII) and Cr(VI) also have been used.
More recently there has been described the oxidation of both free and complex cyanide in aqueous streams by a mixture of sulfur dioxide or alkali/alkaline earth metal sulfites (including bisulfites and metabisulfites) and air or water in the presence of a water-soluble copper(II) catalyst at a pH between 5 and 12; U.S. Pat. No. 4,537,686. [Although copper is designated as "Cu.sup.+ " in the issued patent, the fact that most cuprous salts are water insoluble and that Cu(I) is readily oxidized strongly suggests that Cu(II) actually was used.] Using rather high weight ratios of copper to cyanide on the order of about 0.25, final cyanide concentrations could be reduced to under 0.1 parts per million. More recently Chen et al. (Paper 81c presented at the 1990 AIChE Summer National Meeting, San Diego, Calif., Aug. 21, 1990) presented data on the oxidation with air of aqueous streams containing cyanide at 100 ppm using a soluble copper catalyst in conjunction with sodium sulfite at an optimum pH of 8 over activated carbon in a trickle bed reactor at normal pressure. Initially the copper/cyanide molar ratio was about 0.25, but since copper(II) hydroxide precipitated on the carbon surface, it was found that a copper/cyanide maintenance ratio of about 0.1 was quite adequate. Although the authors characterize the activated carbon as a catalyst, this conclusion is far from clear according to the data. Thus, although the authors showed that use of a bed of activated carbon leads to 99% removal of cyanide, beds of both a molecular sieve and glass beads were almost as effective in affording about 80% removal. The improved result with activated carbon could readily be attributed to adsorption (rather than oxidation) on the bed of activated carbon--activated carbon is known to be an excellent adsorbent--or to the differing extent of copper(II) deposition on the packed beds and its dispersion on the bed materials, or to some combination of the two.
A continuous method for the removal of cyanide using air or oxygen as the oxidizing agent at ambient temperatures and pressures is highly desirable. Although the foregoing references provide a start, much remains before a commercially viable system is operative. In particular, it is often desirable that the catalyst either be heterogeneous, or if homogeneous readily separable, in order to avoid contamination of the effluent by the catalyst itself as well as to minimize process cost associated with catalyst consumption. It also is desirable that the catalyst be relatively insensitive to as large a class of contaminants likely to accompany cyanide as is possible. The process should be capable of efficient operation at atmospheric pressure and preferably as close to ambient temperature as possible in order to minimize energy requirements. Finally, it is desirable for such a process to oxidize the cyanide over a rather wide range of initial cyanide concentrations, and to have the capability of oxidizing 90% or more of the cyanide present.
U.S. Pat. No. 5,120,453 provides a process for the oxidation of cyanide in aqueous streams where the cyanide is present as the anion, CN.sup.-, and where the oxidation is performed under basic conditions. It may be noted that the cited prior art also emphasizes cyanide oxidation under basic conditions, and it also may be noted in passing that basic conditions either are a prerequisite to, or materially enhance the concentration of cyanide ion, so that the correlation between prior art oxidation under basic conditions and the presence of cyanide ion may be a fundamental one rather than being fortuitous. The patentees of the last cited patent use as catalysts a broad class of metal chelates which can be used either in a soluble or water-insoluble form to afford the opportunity of either a homogeneous or heterogeneous process. The oxidation products were largely carbon dioxide and nitrogen along with varying amounts of cyanate.
What we have now found, quite unexpectedly, is that the same class of catalysts as described in U.S. Pat. No. 5,120,453 also is effective in oxidizing inorganic cyanides under acidic conditions. This is surprising not only in view of the prior art which appears to place heavy emphasis on oxidation of cyanide under basic conditions, but also because the catalysts themselves were previously known as oxidation catalysts for mercaptans only under basic conditions. Thus, both relevant prior art relating to cyanide oxidation and relevant prior art relating to the catalysts themselves suggested that basic conditions were necessary for oxidation.
Oxidation of inorganic cyanides under acidic conditions affords an ancillary advantage of placing the oxidation of complexed inorganic cyanides within the realm of feasibility. It is well known that the cyanide ion complexes strongly with many metals to form stable complexes, e.g., ferrocyanides, whose dissociation constant is so small that the attending low (even miniscule) concentration of free cyanide ion is insufficient for any practical oxidation. Thus, these complex metal cyanides may for all practical purposes be oxidation resistant. However, these complexes are dissociated in acidic media (to form HCN). In the invention we describe which utilizes oxidation under acidic conditions it then follows that these strong complexes can be effectively and conveniently oxidized, in contrast to the prior art. Thus our invention now opens the possibility of the direct oxidation of many cyanide complexes such as result from mining and electroplating operations. This will be elaborated on more fully within.
Yet another incidental but significant benefit from oxidation of cyanide under acidic conditions is that the products are CO.sub.2, N.sub.2, and ammonium ion, NH.sub.4.sup.+. Under basic conditions the products are CO.sub.2, N.sub.2, and cyanate, NCO.sup.-. Although the cyanate anion formed under basic conditions is relatively benign, nonetheless the ammonium ion formed under acidic conditions is environmentally far more preferable. Thus the overall result is that the oxidation products formed under acidic conditions are environmentally more benign than those products formed under basic conditions. Consequently acid oxidation of cyanides--especially where the cyanide-containing stream initially is acidic--is environmentally superior to basic oxidation of cyanides.